This Application claims priority over Japanese Patent Application Ser. No. 115367/77 filed Sept. 26, 1977.
The present invention relates to self-adhesive silicone rubber compositions curable at comparatively low temperatures and firmly adhesive to a substrate such as metal or plastics without use of any primer.
Silicone rubbers that cure through the reaction of hydrosilyl radicals and vinyl radicals attached to silicon atoms are well known. This type of silicone rubber has good heat resistance and electrical insulation. Silicone rubber of this type mixed with a filler such as silica, has self fire-extinguishing properties and has been used for potting of electrical and electronic parts and for impregnating coils.
However, these silicone rubbers, when used for potting compounds for electrical and electronic parts, have a tendency to cause corrosion or break down of insulation of the parts since they have no adhesion to the substrate permitting moisture to enter the spaces between the part and the silicone rubber.
Consequently, various types of primers have been developed to cause the silicone rubber to adhere to such electrical and electronic parts. The use of these primers, however, complicates the process since the steps of treating or drying the primers must be taken into account. Most of the primers use a solvent such as toluene, ethyl acetate, and/or alcohol which can also harm the surfaces of plastic parts or melt their paints, and they also require special precautions against their inflammability and toxicity.
On the other hand, efforts have been made to make the silicone rubber self-bonding by adding a proper constituent to the original composition when curing the silicone rubber. It has been proposed that a polysiloxane having hydrogen atoms attached to silicon and a trialkoxysilylalkyl radical be added as a third ingredient to the so-called addition reaction-type silicone rubber that forms bridges and cures by the addition reaction of polyorganosiloxane having vinyl radicals with polyorganosiloxane with hydrosilyl radicals as set forth in Japanese Pat. No. 16952, issued 1973. Also, a silane or siloxane with acryloxyalkyl radicals, and an organic peroxide are added as set forth in Japanese Pat. No. 26855, issued 1975, and further a polysiloxane having epoxy groups and/or ester groups attached to silicon atoms, and hydrogen atoms directly attached to silicon atoms, as set forth in Japanese Pat. No. 39345, issued 1975, are added to create self-bonding properties in the composition. None of these compositions exhibit sufficient self-adhesion to various types of substrates used for electrical and electronic parts.
As a result of efforts to improve these defects, it has been found that an unsaturated hydrocarbon compound with oxirane radicals added as a third ingredient to a silicone rubber composition curable by reaction of hydrosilyl radicals with alkenyl radicals attached to silicon atoms makes the composition self-bonding to a metal or plastics by heating at comparatively low temperatures for short periods of time. This method has, however, a tendency to yield unclear compositions due to poor solubility with the organopolysiloxane and oxirane compound, and to produce compositions with less self fire-extinguishing properties. Upon completion of further research, the inventors have proposed that an organopolysiloxane with oxylene radicals, trialkoxysilyl radicals, and hydrosilyl radicals be added as a third ingredient to improve these defects--See Japanese Patent Application No. 107459, 1976. This method provides excellent self-adhesion when the composition is heated at 100.degree. C., but unsatisfactory self-adhesion at a low temperature of 70.degree. C. so that its application is limited to the substrates or parts able to withstand a temperature of 100.degree. C.